The luminal endoprostheses which have been developed to date generally assume simple tubular or cylindrical shapes or, more rarely, a hyperboloid shape. Such prostheses are described in particular in the documents WO-A-83/03752 and GB-A-1 205 743.
These prostheses include a tubular braided structure for an arterial conduit or other conduit and are put into place, after compression of their diameter, using a tubular applicator. These prostheses are not intended for implantation in a bifurcated shape.
Attempts have been made to develop bifurcated shapes of prostheses. These essentially concern the junction of the abdominal aorta, which is a large vessel, but investigations are in their infancy.
A bifurcated prosthesis for implantation in a conduit such as a blood vessel is known, in particular, from U.S. Pat. No. 4,994,071.
This prosthesis, made up of interconnected metal rings, comprises a trunk formed by a first series of interconnected rings, and at least one branch formed by a second series of interconnected rings, these at least two series being connected to each other via a flexible element, folded in such a way as to correspond to the angle between the ramifications in question. Putting such a prosthesis into place is extremely difficult, or even impossible.
U.S. Pat. No. 5,609,605 describes a bifurcated endoprothesis made out of two single balloon endoprostheses of variable diameter placed side-by-side in a bifurcated lumen.
Documents EP-A-0 539 237 and WO 96/34580 describe devices for putting bifurcated endoprostheses into place. These endoprostheses include a main body (trunk) and two members (branches) extending from the main body; they are made of woven, folded or pleated fibre. Placing such endoprostheses in arterial bifurcations is a long and delicate operation, especially on account of stringent requirements in respect of orientation and positioning. Furthermore, the angle provided by the two branches does not necessarily correspond to the original angle between the vessels.
Document EP-A-0 461 791 illustrates the difficulties involved in putting a bifurcated prosthesis into place, even in the case of an aneurysm of the abdominal aorta, where the vessels are of a large diameter.
Other documents as U.S. Pat. No. 5,609,627 and U.S. Pat. No. 5,639,278 describe trouser-form endoprotheses with added legs, which often cause blood flow perturbation.
The bifurcation angles differ from one person to the next and from one population to another. They are smaller and more asymmetrical in the elderly than in younger subjects. The bifurcations are more asymmetrical in men than in women. Comparative studies have shown that the bifurcation angles in Asiatics are wider than in Caucasians.
Hydrodynamics also teaches that the thicknesses of the arterial walls differ from one population to another. When these walls are thin, the effort necessitated by the transport of the blood increases. It is also known that when the vessels are too large, the volume of blood increases beyond what is necessary. These factors promote aneurysms (dilation of the arterial wall).
The considerations detailed above show that it would be necessary to adapt the design of a bifurcation to each anatomical site, and also that this design must take into account the differences between different types of populations, between men and women, between the young and the elderly, etc. In practice, it is not possible to provide bifurcations tailored to each patient. This would in fact risk causing problems associated with waiting periods and prohibitive costs.